The ability to repair rather than replace gas turbine engine aerofoil components such as turbine blades and vanes which become damaged in use is an important factor in reducing engine operating costs for the cost of new components is very high. Such components are routinely found to have incurred damage eg by cracking or severe corrosion, to a degree which requires repair and is indeed repairable providing that surface contamination can be removed. Blades and vanes from the hot section of the engine are particularly prone to oxide and corrosion contamination because these components experience more arduous conditions than others. This contamination is especially difficult to remove from cracks and other tight spots.
Many processes have been used over the years to remove surface contamination from engine run components in order to provide a clean surface which is necessary for brazing or welding repair. For earlier turbine component alloys techniques utilising abrasive blasting, wet processing or reduction at high temperature by hydrogen have proved adequate. However new alloys have been developed to accommodate more arduous operating conditions and such of these alloys as contain for example aluminium, titanium, niobium, hafnium or yttrium are not susceptable to cleaning by the above mentioned techniques because the surface contamination is more tenacious and stable. An alternative technique for these newer alloys utilises fluoride-based reactants.
Keller et al in U.S. Pat. No. 4,098,450 describes a fluoride-based cleaning process using a gaseous reactant produced from a fluoride powder subjected to a high temperature hydrogen atmosphere in a retort. The component to be cleaned is subjected to the action of the reactant at a temperature in the range 870.degree.-1100.degree. C. The resultant reaction converts the metal oxide of the component to fluoride compound which has some degree of volatility and tends to evaporate from the component surface to be carried away by a stream of the reactant gases.
In U.S. Pat. No. 4,188,237 and 4,324,594, Chasteen describes another fluoride-based cleaning process. In this process polytetrafluoroethylene (PTFE) is decomposed at high temperature within a stream of hydrogen and the resultant atmosphere is used to clean components by conversion of metal oxides to fluoride compounds and evaporation of the latter.
Both these prior art fluoride-based cleaning process utilise a continuous flow of reactants. This flow is apparently caused by inlet pressure and from this it is deduced that the process is performed at atmospheric pressure or greater and under steady flow conditions.
The Applicant has found that brazed repairs made to engine run superalloy components and also to artificially damaged superalloy test pieces, made after cleaning thereof by the prior art fluoride-based cleaning process are often insufficiently effective in their pentration of fine surface cracks. This lack of effectiveness is believed to stem from inadequate oxide removal from the interior of the cracks rather than failure of the braze metal to enter such cracks once adequately cleaned.
In order to illustrate the deficiencies of the state of the art cleaning process, reference is made to FIG. 1 of the drawings which shows three micrographs A,B and C, of inadequate braze repairs made to superalloy engine components in consequence of insufficient removal of oxide from the internal surfaces. The specimens shown were cleaned and repaired by a recognised expert source. All specimens shown have been cleaned using a halide based process. The following comments are addressed to the individual micrographs.